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B.1.2 For the purposes of this Annex, the expression "Unprotected area" has the meaning ascribed to it by Cl.1.4.86. but in calculating the size of unprotected areas or the permitted limit of unprotected areas, the following provisions shall apply:

a. where any of an external wall is an unprotected area, only because it has combustible material attached to it as cladding, the area of that unprotected area shall be deemed to be half the area of such cladding;

b. no account shall be taken of any of the following:

(1) an unprotected area which does not exceed 0.1m2 and which is not less than 1.5 m from any other unprotected area in the same side of the station or compartment (unless that other falls within (3) below;

(2) one or more unprotected areas having an area (or if more than one an aggregate area) not exceeding 1m2 and not less than 4 m from any other unprotected area in the same of the station or compartment (except any such area as is specified in B.1.1 above);

(3) an unprotected area in any part of an external wall which forms part of a protected shaft; and

(4) an unprotected area in the side of a station not divided into compartments, if the area is not less than 28m above any ground adjoining that side of the station.

B.2.2 For the purpose of this Part of this Annex.

"relevant boundary" means as defined in Cl.1.4.72 and for the purpose of this calculation is either paralleled to the side of the station under consideration or at an angle of not more than 80o with that side;

"plane of reference" means any vertical plane which touches the side or some part of the side of a station or compartment but which (however far extended) does not pass within the structure of such station or compartment (and for this purpose , any balcony , coping or similar projection shall be deemed not to be part either of that side or of the structure) ; and the relevant plane of reference shall in each case be taken as that most favourable in that respect to the person erecting the station;

"enclosing rectangle" means the smallest rectangle on the plane of reference which would-

a. enclose all the outer edge of any unprotected area of the station or, if the station is divided into compartments, of the compartment (other than any of an unprotected area which is at an angle of more than 80º to the plane of reference by line perpendicular to such plane;

b. have two horizontal sides; and

c. have height and width falling within those listed in the Tables of this Annex.

"unprotected percentage" means the percentage of the area of the enclosing rectangle which is equal to the aggregate of the unprotected areas taken into account in calculating the enclosing rectangle and as projected on it.